Simple Dragon Eggs

[Bju90]

I came across a simple crackle comp but I am not sure about binding it with dextrin?

70% Bismuth Trioxide

30% Magnalium 200 Mesh

Bound with 5% dextrin

 

I can't find the original place I found this but it did quote this being an early comp used for crackle but substituting out the lead for bismuth. However I can't find anywhere else this has been used. 

I tested this as an unbound powder while I am waiting for my copper oxide and NC to turn up and it certainly produced an energetic crackle type reaction. 

I wasn't sure about binding with dextrin though. iirc it was with alcohol but this confuses me because dextrin isn't soluble in alcohol. 

[Zumber]

Just don't try to bind crackling composition with water & dextrin.

Just wait until you get NC lacquer and cupric oxide.

Try to use tested formulations and stay safe.

[Bju90]

I have heard to not use water. Is that because of the potential for an exothermic reaction of water with the magnalium? Does it apply to other magnalium compounds or is it because of the high magnalium content in crackling eggs?

I wasn't sure if removing the Copper Oxide decreases reactivity so it becomes less sensitive.

I appreciate the sentiment of using tested formulations... but my neurodivergent brain  has trouble following a rule unless I understand the why. Plus it's interesting understand the underlying mechanics/chemistry.

[Almostparadise]

1 hour ago, Bju90 said:

I have heard to not use water. Is that because of the potential for an exothermic reaction of water with the magnalium? Does it apply to other magnalium compounds or is it because of the high magnalium content in crackling eggs?

I wasn't sure if removing the Copper Oxide decreases reactivity so it becomes less sensitive.

I appreciate the sentiment of using tested formulations... but my neurodivergent brain  has trouble following a rule unless I understand the why. Plus it's interesting understand the underlying mechanics/chemistry.

If you do bind the above composition w dextrin and water please let us know how it turns out!

[SKC]

10 hours ago, Bju90 said:

I have heard to not use water. Is that because of the potential for an exothermic reaction of water with the magnalium? Does it apply to other magnalium compounds or is it because of the high magnalium content in crackling eggs?

I wasn't sure if removing the Copper Oxide decreases reactivity so it becomes less sensitive.

I appreciate the sentiment of using tested formulations... but my neurodivergent brain  has trouble following a rule unless I understand the why. Plus it's interesting understand the underlying mechanics/chemistry.

Get success first with this very simple crackle mix.

Pb3O4-75

MgAl-15

Cu2O-10

Resembles with classic BP mix, right?

But bind with NC lacquer. You can tweak this by adding % of Ti. This composition produce loud sound when bound with lacquer as the granules becomes more dense and harder compared to dextrin bound granules. 

Lacquer prepared with acetone prevents any exothermic reaction when the composition is mixed. So it is some kind of safer to use this formulation.

Don't forget to wear gloves while working with this crackle composition.

[Bju90]

This paper was interesting. Although I have a hunch that NC Lacquer volume that he wasn't able to control effectively would have a significant impact on the results of some measures.

The original formula I have shared seems to fall quite a way off the stochiometric ratio, assuming Pb and Bi are a direct swap doesnt change this significantly. It would be more like:

89% Bismuth Trioxide

11% Magnalium 200 Mesh

Bound with 5% dextrin Can't find any report of success using dextrin so I won't even bother. Water soluble Phenolic Resin though?

 

I'd be interested to see if any one has similar experience using a higher ratio of CuO with respect to the delay time, 12s seems a bit absurd and very impractical? I can get CuO much easier/cheaper so it would be favourable if a higher CuO composition does what I want it to. 

 

DragonEggStudy.pdf

[AustralianPyromaniac]

The mechanism is extremely complicated and in fact is not fully understood. The delay is reliably changed by changing mesh size of the MgAl, not by adding CuO. The reaction is probably a physically delayed high-speed thermite reaction, which can be accelerated by low melting and boiling point metal oxides such as Bi and Pb. My own ideas are written below, but nothing is known for sure. 

---

BURNING/ SMOLDER (BELOW AL MELTING POINT)

The reaction proceeds in phases, as has been established by Shimizu. The first stage is smoulder, where the metal oxide is reduced in the flame to the form ME₁O₁ ME₂O₁ and ME (ME meaning any metal), where the metal is reduced from higher oxidation states to the low +1 and +2, and in some cases even further to the metallic form. The oxygen removed in this stage forms particles of aluminium coated in aluminium oxide. Research has shown that this coating does develop and is it reasonable to infer this slows the reaction rate. The coating of oxide prevents further rapid reaction between the metal oxide and the aluminium. As the aluminium oxide is formed, it sinters together forming relatively “large” (50 um), embedded in which is the aluminium. During this stage, oxygen free radicals are also created. These radicals roam through the aluminium oxide matrix, diffusing into the alumina, and continuing to react with the aluminium raising the temperature of the star.

It is hard to explain why magnesium alone does not work for this reaction. It is likely that is too reactive. This is to say, even with a passivating coat of MgO, the reaction cannot be arrested, and continues to oxidiser very rapidly without a smoulder phase, i.e. flash powder. Most other properties of magnesium are the same as aluminium. The melting point of magnesium is very similar to that of aluminium. Its boiling point is lower than that of aluminium, approx. 1000c, but this temperature is not theorised to be reached in this stage of the reaction.   

EXPLOSIVE TRANSITION

Once the melting point of the aluminium is reached (660c), the star stops heating further and must pass the first exotherm (all the generated heat goes to melting the aluminium). As the aluminium core particles melt, they remain surrounded by a layer of solid aluminium oxide, which has an extremely high melting point. During this stage the start is extremely sensitive to shock, and this has been established by Shimizu’s research. All that is stopping a violent runaway reaction at this point is the thin shell of alumina. If it cracks, the liquid aluminium will react extremely quickly with the surrounding oxide an explode.

Once all the aluminium has melted, the star is again free to increase in temperature, and the temperature continues to rise as more oxygen diffuses into the alumina shell and reacts.

At some point, a localised breakdown of one or more alumina shells occurs. This is to say, aluminium seeps out into the bulk oxide and reacts near instantly to generate Cu metal and Al2O3. The liquid phase of the aluminium causes this reaction to occur extremely rapidly. A shock wave, and a large amount of heat is generated by this reaction. If the mixture is made well, the spread of this shock wave breaks the remaining alumina shells and the star detonates. This is a true detonation, above the speed of sound, and propagated by shock not heat diffusion.

A 2019 paper studying CuO-Al pyrotechnic crackle found an unreliable explosive transition temperature, ranging from around 700-1000 C. That is to say, the crack occurred when the smouldering phase had brough the star somewhere between 700 and 1000 C. This transition always occurred after the melting of aluminium, but before the boiling of the metal oxide. Shimizu, in his article in Pyrotechnica attempted to measure the transition temperature of the explosion of lead oxide crackle, and was only able to collect 1 data point, 850C, which is in line with this theory, and does not support the metal oxide boiling theory he ended up accepting.

EXPLOSION

Once the alumina shells break and liquid aluminium contacts the bulk oxide, the reduction reaction occurs near instantly and everything explodes. This liberates enormous amounts of heat. Research has shown CuO-Al thermite reactions to peak at over 3400 C. Thermite reactions are generally understood to create no gassous products. In the case of explosive thermites, we know this is not the case. CuO-Al thermite theatrically has no gaseous end products, but burning this mixture shows a definite “plume” of smoke characteristic of rapid gas generation. Confined CuO-Al is capable of creating a pressure wave which can burst a steel container. This supports the generation of products in vapor form. Aluminium oxide has a boiling point of 3000c and is unlikely to be vaporised. Bismuth and lead metal have very low boiling points and low heat of vaporisation, so are likely contribute to the explosive effect of these thermites by boiling. Copper has a high boiling point, and moderate heat of vaporisation, but is also an extremely energetic thermite. Gram for gram, copper thermite releases 3 times more energy than lead thermite, and 2 times more energy than bismuth thermite. This energy contributes to the boiling of the bismuth and lead metal, which is what causes the explosion. Copper thermite alone, while very energetic, does not have the energy to boil its metallic products to as great a degree.

[Bju90]

Interesting, thanks for sharing! I’ll have to read it again once I’ve slept but I get the gist of it. 
 

Tested the above simple 89/11 formula and it produced a result. 

 

From what you have written, am I right in assuming the addition of CuO to this would make for much more energetic explosion?

And the formula floating around with a bit of additional aluminium to push the ratio in favour of aluminium over mg, would this slow the first stage and result in an increased the delay? Or is it more to reduce sensitivity?